Thermophotovoltaic (TPV) energy conversion involves the conversion of heat to electricity, and has been identified as a promising technology since the 1960's. A basic TPV system includes a thermal emitter and a photovoltaic diode receiver. The thermal emitter is typically a piece of solid material or a specially engineered structure that generates thermal emission when heated to a high temperature (i.e., typically in a range from about 1200° K to about 1500° K). Thermal emission is the spontaneous radiation (emission) of photons due to thermal motion of charges in the thermal emitter material. For normal TPV system operating temperatures, the radiated photons are mostly at near infrared and infrared frequencies. The photovoltaic diode receiver includes a photovoltaic (PV) cell positioned to absorb some of these radiated photons, and is constructed to convert the absorbed photons into free charge carriers (i.e., electricity) in the manner typically associated with conventional solar cells. In effect, a solar energy system is a type of TPV energy conversion system where the sun acts as the thermal emitter. However, the present invention is directed to “earth-bound” TPV energy conversion systems in which the thermal emitter is solid structure that is heated from an external source (e.g., by concentrated sunlight or other heat generator).
Although TPV energy conversion is promising in theory, practical conventional TPV systems have achieved far lower efficiencies than theoretically predicted. A TPV system can be modeled as a heat engine in which the hot body (i.e., the heated thermal emitter) is described as a blackbody radiation source having a black body temperature TBB, and the relatively cold PV receiver has a temperature TPV, whereby the theoretical thermodynamic efficiency limit is given by the Carnot cycle ηCarnot=(TBB−TPV)/TBB. For a thermal emitter temperature TBB equal to 1500° K and a PV receiver temperature TPV equal to 300° K, a theoretical efficiency ηCarnot equals 0.8 (80%), which exceeds the Shockley-Queisser limit (i.e., the maximum theoretical efficiency of a solar cell using a p-n junction to collect power). In reality, however, the efficiencies of conventional TPV systems are reported to be below 10%. This is believed to stem from a mismatch between the spectrum of the thermal emitter and the PV cell.
One reason for the lower realized efficiencies of conventional TPV systems is related to carrier thermalization at high temperatures caused by a mismatch between the emitted photons and the PV cells. Thermal radiation from the thermal emitter (hot body) of a TPV system has a spectral power density given by Planck's law, and the peak wavelength λmax is given by Wien's law (λmax˜(2898/TBB) μm). For high-temperature emitters (1100° K≦TBB≦1500° K), the peak wavelength λmax is in the range of 1.9 to 2.6 μm, which requires the TPV system to utilize PV cells having low bandgap semiconductors (i.e., around 0.5-0.8 eV). Using such low bandgap PV cells requires the use of emitter materials having bandgaps closer to 0.5 eV (˜2.5 μm) in order to obtain a larger fraction of in-band photons at reasonable emitter temperatures (i.e., 1100-1500° K). If emitter materials having bandgaps below 0.5 eV are used, the PV cell performance suffers from high carrier thermalization at the elevated temperatures required in TPV systems.
What is needed is a thermophotovoltaic (TPV) converter that overcomes the problems set forth above and converts heat energy to electricity with a much higher efficiency than achieved using conventional TPV approaches. What is particularly needed is a TPV converter that achieves efficiencies of at least 25% with a wide variety of different PV cell types.